Premium medical diagnostic ultrasound imaging systems require a comprehensive set of imaging modes. These are the major imaging modes used in clinical diagnosis and include timeline Doppler, color flow Doppler, B mode and M mode. In the B mode, such ultrasound imaging systems create two-dimensional images of tissue in which the brightness of a pixel is based on the intensity of the echo return. Alternatively, in a color flow imaging mode, the movement of fluid (e.g., blood) or tissue can be imaged. Measurement of blood flow in the heart and vessels using the Doppler effect is well known. The phase shift of backscattered ultrasound waves may be used to measure the velocity of the backscatterers from tissue or blood. The Doppler shift may be displayed using different colors to represent speed and direction of flow. In the spectral Doppler imaging mode, the power spectrum of these Doppler frequency shifts are computed for visual display as velocity-time waveforms.
one of the primary advantages of Doppler ultrasound is that it can provide noninvasive and quantitative measurements of blood flow in vessels. Given the angle .theta. between the insonifying beam and the flow axis (hereinafter referred to as the "Doppler angle"), the magnitude of the velocity vector can be determined by the standard Doppler equation: EQU v=cf.sub.d /(2f.sub.0 cos .theta.) (1)
where c is the speed of sound in blood, .function..sub.0 is the transmit frequency and .function..sub.d is the motion-induced Doppler frequency shift in the backscattered ultrasound signal.
In conventional ultrasound scanners that perform B-mode and spectral Doppler imaging either simultaneously or in a segmented fashion, the angle between the Doppler beam cursor (beam centerline) and a vessel slope cursor in the B-mode image is used to convert Doppler frequency shifts into velocity units according to the Doppler equation. The operator is required to manually adjust (e.g., via a toggle switch) the vessel slope cursor based on the orientation of the vessel wall(s) in the B-mode image. The Doppler angle value is usually displayed along with the graphic. Since the Doppler angle adjustments are based on visual judgment, they are susceptible to error, especially if the angle step size is coarse. If fine angle adjustments are possible, the process can become time consuming. Therefore, an automatic method of adjusting the vessel slope cursor is needed to improve both the accuracy and efficiency of Doppler velocity measurements.
European Published Patent Application EP 0842638 teaches a method for automatically tracking the vessel walls, thereby enabling vessel diameter and volume flow measurements. However, the method requires the operator to first manually position the wall lines of a special cursor until they are coincident with the near and far vessel walls. In the middle of the special cursor lies the vessel slope line which is parallel to the wall lines.
U.S. Pat. No. 5,690,116 teaches a method for automatic measurement of the Doppler angle and an arrangement for carrying out the method. This method consists of the following basic steps: (1) From an initial point, a first isotropic tracing of rays over the region of interest is performed so as to provide a histogram of gray levels of selected points along the rays. (2) An image processing algorithm is executed on the histogram which results in a lower threshold for detecting vessel wall echoes. (3) A second tracing of rays from the initial point is performed, during which the gray level of each point of each ray is compared with the threshold, and the first end point of each ray whose gray level exceeds the threshold is classified as an edge point. This results in a representation of the blood vessel in the form of a so-called "local mark" of triangular sectors. (4) The slope of the regression line of the local mark is determined and the Doppler angle between the fitted line and the Doppler (beam) cursor is calculated. Since a linear regression of the coordinates of all the pixels in the local mark is performed, this prior art method implicitly assumes that the pixels representing the near and far vessel walls are both clear and reliable. The validity of the resultant slope estimate is tested by checking the coefficient of correlation. A correlation below a certain acceptable level (e.g., 0.5) is held to be indicative of poor positioning of the sectional plane, and the operator must make another attempt after correction.
In routine clinical examinations, even if the scan plane is aligned properly with the central axis of the vessel, often one of the two walls may be corrupted or masked by reverberation noise and/or shadows. Sometimes a nonlinear gray map is used to improve the perceived contrast of the image display. For these reasons, using a gray-level-only threshold alone can lead to false detection of edges. Furthermore, it is not uncommon that one of the two vessel walls will not appear clearly in the image simply because of the scan geometry in relation to the curvature of the vessel. In some cases the near and far walls may not even be parallel in the best vessel image that can be obtained in the time allowed. If a Doppler velocity measurement still needs to be carried out in any of the above situations, the user will often align the vessel slope cursor to the more clearly defined vessel wall, or make the best judgment by trying to "see through" some of the clutter in the vessel image. To automate the Doppler angle estimation in such challenging situations, a more robust method than that known in the prior art is needed.